Paint Glows Under Pressure

Researchers at Purdue University have mixed fluorescent marker molecules and binders to create paints that indicate changes in temperature and pressure by changing the intensity of their fluorescence. The technique has advantages over conventional point measurement systems because it is nonintrusive and measures an entire painted surface. Applications include wind tunnel testing as well as engine and rotor testing.

Temperature- and pressure-sensitive paints on rotors turning at 17,800 rpm indicate a wide temperature difference and the presence of a shock wave, which can decrease engine efficiency.
"The concept is to use molecular probes for mechanical measurements," said John Sullivan, Purdue professor and head of the school of aeronautics and astronautics, who has been working on the method for several years. Researchers, led by Sullivan, mix fluorescent markers such as rhodamine B, porphyrins or rutheniums with binding agents. The molecules fluoresce when illuminated with light of a specific wavelength, usually in the ultraviolet.

As the temperature increases, the molecules lose some of their fluorescence, because part of the energy they absorbed from the excitation light is lost through nonradiative decay. Likewise, in the presence of oxygen they can lose their fluorescence by transferring their absorbed energy to oxygen molecules where it becomes vibrational energy. As the air pressure increases, more oxygen is available to the molecules, and they fluoresce less. When the mixture is painted on an airplane wing, helicopter rotor or the blade of a gas turbine engine, the researchers can watch temperature and pressure distributions develop and change.

The typical system includes a charge-coupled device camera or a photomultiplier tube for detection and a laser, such as a frequency-doubled Nd:YAG, for illumination. Sullivan said applications that are more precise require more expensive hardware, but "even then you would be hard-pressed to spend more than $50,000." His experiments include temperatures from ­193 to 300 °C. He explained that his research group has used some 25 materials with different temperature ranges and sensitivities.

Wind-tunnel testing

The technique has the potential to supplant conventional pressure and temperature testing in wind tunnels because it avoids point detection and may cost less. Sullivan added that an airplane manufacturer may spend upward of $1 million to set up a wind tunnel to test its products with conventional point detection systems. For other applications such as measuring temperature and pressure in a gas turbine engine, it is virtually impossible to use thermocouples and pressure sensors.

"The temperature-sensitive paint is usable right now for [temperature] transition detection across a wide range of temperatures," Sullivan said. "We think we can do quantitative heat transfer in many ranges, too. For pressures, semiquantitative measurement, within 10 percent, is fairly easy." Increasing the pressure measurement accuracy, however, is difficult because the pressure-sensitive paints also are affected by temperature as well as by environmental dirt and oil.

The researchers' plans include working on unsteady flows such as those that occur on an airplane as it maneuvers. Sullivan added that he has received inquiries from golf ball manufacturers and race car teams. He also plans to paint the wings of Purdue's business jet to make in-flight measurements.